Electricity generation is the process of converting the power from an energy source into electricity. Sources of energy can include renewable energy sources, derived from natural processes that are replenished constantly, including, for example, sunlight, wind, rain, tides, waves and geothermal heat. There are a number of renewable generation facilities that use electricity generators to convert the power from an energy source into electricity, including solar photovoltaic (PV) cells, solar power generators, thermoelectric generators, tidal electricity generators, wind power generators/turbines, and other electric generators using any other source of energy. Renewable generation facilities can be appropriately connected to deliver power to a load, an electrical grid for further distribution, and/or an energy storage device/system that banks electrical power (such as pumped hydro, compressed air, flywheels, and battery energy storage, as known in the art).
Specifically, power can be generated via a wind-farm wherein the electricity generators are wind turbines used convert the power in the wind to electricity. Wind farms are created when multiple wind turbines are placed in the same geographic area for the purpose of generating large amounts of electrical power for delivery to a utility grid. Recently, the fraction of energy added to utility grids by wind farms has increased dramatically. Therefore, there is a need for controlling wind farms delivering energy to utility grids with respect to a number of parameters prescribed by the utility grid operators.
Wind power is variable because wind speed is necessarily variable. Generally, wind power varies as a cube of wind speed. Consequently, even relatively small changes in wind speed result in significant changes in wind power. For example, an increase in wind speed from 9 m/s to 10 m/s may result in an increase of wind power of almost 40%. Wind gusts can cause even more of a concern. Sometimes this increase in wind power exceeds a maximum allowable variation in wind park output prescribed by transmission system/utility grid operators, particularly in small power systems. For example, in systems like Hawaii, operators require that the plant MW output not vary by more than a certain +/− change in MW from the output measured “n” seconds earlier.
Attempts have been made to control wind farm power including control schemes for controlling power ramp rate of a wind farm, such as that described in U.S. Pat. No. 7,679,215. The wind farm power rate control method described therein attempts to limit the rate of change of collective power output by generating a power output rate limiting signal based on the monitored rate of change of collective power output of the wind turbine generators and a desired collective power ramp of the wind turbine generators and applying the power output rate limiting signal to the plurality of wind turbine generators. In operation, when the actual wind farm power ramp rate increases above the ramp rate request, the controller adjusts the wind farm ramp rate, causing actual power output to fall below the ramp rate request. As a result, the arithmetic sum over a one minute time duration is zero. However, although this scheme provides for an “average” ramp rate that coincides with a ramp rate request, the wind farm power still exceeds the maximum allowable plant output power during this time frame and, thus, fails to meet the requirements of grid operators for the output to not vary by more than a certain +/− change in power from the output measured “n” seconds earlier.
Some disadvantages of this scheme include: ramp rate limits can be exceeded for a short period of time; non-optimal power production due to the fact that the power is being controlled by a turbine ramp rate instead of a turbine power setpoint; and, finally this control requires a ramp rate control in the wind turbines.
In view of these variations in wind power that cause the output power to vary beyond prescribed limits and the shortfalls of the prior attempts, there is a need in the art for a system and method to limit the output power to comply with the transmission system/utility grid operators' requirements. There is a further need in the art to provide a control mechanism that performs an output control function to bound the variation in wind park output to comply with the transmission system/utility grid operators' requirements. There is a further need in the art for a system and method to maintain frequency and load-generation balance.
The present invention is designed to address these needs.